The invention relates to a low-frequency amplifier with an integrated push-pull class B final stage and a circuit for adjusting the quiescent current.
The permissible operating voltage range required for integrated low-frequency push-pull class B amplifiers is becoming progressively larger. The reason for this is that these low-frequency amplifiers are to be used for all kinds of equipment, for example, for portable sets with headphones as well as for medium-sized and larger equipment, with power supply overvoltages being taken into consideration, requires operating voltages of up to 16 V.
A further requirement exists with respect to adjustment of the quiescent current. As small a quiescent current as possible is desired in order to keep the load on batteries low. Crossover distortions can, however, occur with quiescent currents which are too low and, therefore, a compromise must also be sought with respect to adjustment of the quiescent current. The output dc voltage of the low frequency push-pull class B amplifier should have the optimum value required for maximum low-frequency output power. The output dc voltage should, therefore, correspond approximately to half of the operating voltage.
Two suitable low-frequency amplifier circuits which meet some of the above-mentioned requirements are known from German Patent No. 3,409,417. These known circuits are shown in FIGS. 1 and 2 and, for better understanding, are explained in detail hereinbelow.
The final stage shown in FIG. 1 comprises transistors T12 and T13 of the same conduction type. The lower final stage branch with transistor T13 is driven via a complementary transistor T11 which simultaneously effects the phase reversal. The base of this complementary transistor T11 is connected to the collector of driver transistor T1. The collector of the driver transistor T1 is connected via diode D2 and current source Q3 to supply voltage U.sub.s. The voltage at point M constituting the output of the final stage is adjusted to half of the battery voltage by a control circuit which is not illustrated. This point M is connected to the junction between the final stage transistors T12 and T13 and constitutes the output of the amplifier circuit with the load R.sub.L.
In the circuit shown in FIG. 1, the adjusting circuit for the quiescent current comprises diode D2, across which a voltage drop U2 is generated, and diodes D7 and D8 across which corresponding voltage drops U7 and U8 are generated. The anode connection of the diode series circuit comprising diodes D7 and D8 is at the base electrode of transistor T10, complementary with transistor T11, and is further connected via a current source Q9 to the positive pole of supply voltage source U.sub.s. The cathode of diode D8 is at point M and hence at the output of the final stage amplifier. The emitter electrodes of transistors T10 and T11 are interconnected. The collector of transistor T10 is connected to the positive pole of the supply voltage source. The junction point between source Q3 and diode D2 is connected to the base electrode of a transistor T4 whose emitter is at M and whose collector branch contains diode D5, across which the base-emitter voltage for transistor T6 drops. The collector of transistor T6 is connected to the control or base electrode of the final stage transistor T12 and can be connected to M via a resistor R23. Diodes D2, D7 and D8 together with the base emitter paths of transistors T4, T10 and T11, form a voltage loop. The sum of the voltages at the base-emitter paths of these transistors consequently coincides with the sum of the voltages at diodes D2, D7 and D8. The voltage drop across diodes D2, D7 and D8 can be adjusted by corresponding selection of the currents of current sources Q3 and Q9 such that the desired current flows in transistor T4 and in transistor T11, respectively, which then predetermines the quiescent current of final stage transistors T12 and T13. On account of the necessary voltage drop across diodes D7 and D8, the supply voltage in the circuit of FIG. 1 must not drop below 3 V as the voltage drop required for operating current source Q9 would otherwise not be large enough. On the other hand, this disadvantage is contrasted with the advantage that the current flowing through diodes D7 and D8 is very small since the base current of transistor T10 is smaller by this current gain factor than the current flowing through collector T11.
In the circuit shown in FIG. 2, the supply voltage can be further reduced. In this circuit, transistor T10 is eliminated and the diode chain consisting of diodes D7 and D8 is replaced by a single diode D14. The anode of this diode D14 is directly connected to the emitter of the complementary transistor T11, whose collector drives the base electrode of final stage transistor T13. The voltage at point M is, therefore, only raised by a diode forward ,voltage U14. Thus the potential at the anode of diode D14 is approximately 0.7 V lower than the comparable potential at the base electrode of transistor T10 in FIG. 1. The voltage loop is formed by diodes D2 and D14 and by the base-emitter paths of transistors T4 and T11. Therefore, the voltage U.sub.s can drop to values of up to 1.8 V as a voltage of approximately 1.6 V is then present at the emitter of transistor T11 and the residual voltage of 0.2 V is just adequate to operate the current source Q15. When the circuit is driven by a signal, a current which increases as the signal increases flows through final stage transistors T12 and T13. Hence the base current of transistor T13 also rises and, consequently, the emitter current of transistor T11. To enable transistor T11 to prepare for this change in the emitter current, the current flowing through current source Q15 and hence also through diode D14 must be chosen relatively large in comparison to the emitter current of transistor T11. Therefore, in the circuit of FIG. 2 the advantage of a reducible supply voltage is contrasted with the disadvantage of the higher current consumption.